This invention relates to nuclear magnetic resonance (NMR) apparatus and more specifically to a combined NMR signal pickup probe and low loss, wide band front end for an NMR receiver.
Nuclear magnetic resonance occurs in atomic nuclei having an odd number of protons and/or neutrons. Such nuclei have a net magnetic moment and precess or rotate when a magnetic field force is applied, and the spin axis of the nucleus is made non-parallel to the field. The rate at which the nuclei precess is dependent on the strength of the applied field and the characteristics of the nuclei. The angular frequency of precession, .omega., is defined as the Larmor frequency and is given by the equation .omega.=.gamma.H, in which .gamma. is the gyromagnetic ratio (constant for each type of nucleus) and H is the strength of the uniform magnetic field. A precessing nucleus is capable of absorbing electromagnetic radiation at the Larmor frequency .omega.. The absorbed energy may be reradiated as an NMR signal, and upon detection analyzed to yield useful information about the material such as its structure, diffusion properties of molecules, and the relative abundance of the material or, in the case of a nonuniform field, information concerning the physical location of the radiating nuclei can be obtained. With proper excitation, detection and reconstruction techniques, the reradiated NMR signals provide sufficient information to construct cross-sectional images of various bodily organs or even cross sections of the entire torso or head. Such images are useful for medical diagnostic purposes. Imaging methods employing NMR offer significant advantages over conventional imaging methods which use ionizing radiation. An exemplary NMR imaging method is described and claimed in allowed application Ser. No. 067,697, filed Aug. 20, 1979 by R. S. Likes, and which has now issued as U.S. Pat. No. 4,307,343, on Dec. 2, 1981, and which is assigned to the same assignee as the present invention.
Since the effective signal-to-noise ratio of the NMR signal can be improved by averaging, there is a trade-off between signal-to-noise ratio and the time required to gather data. However, since the data needed for NMR imaging requires a patient to remain immobilized while it is being collected, it is desirable to minimize the actual data collection time, and it is therefore desirable to maximize the signal-to-noise ratio in every possible way.
The signal-to-noise ratio is generally improved by frequent excitation of the nuclear magnetic dipoles and by maximizing the efficiency of the NMR receiver over the entire frequency band in which they are radiating. Since this frequency spectrum is broadened by the magnetic field gradients which are present in NMR systems, it is usally much broader than the natural bandwidth of conventional pick-up coils (probes). If, in fact, it is desirable to distinguish the NMR signals arising from different spatial positions on the basis of their respective resonant frequencies, it is necessary to apply a magnetic field gradient of sufficient strength to spread out the signal spectrum by a factor of N as compared to the natural frequency of the NMR signal in a homogeneous field (N being the number of image points (pixels) to be resolved in the image). Thus, when many hundreds or thousands of pixels are involved, the signal must be much broader in frequency than the natural bandwidth of conventional probes.
Although it is possible to achieve a broad bandwidth in a conventional probe by applying a dampening resistance, this approach degrades the signal-to-noise ratio. If an efficient probe having a narrow frequency response is used, however, its resonant frequency would have to be swept across the band of interest, and this would add to the time required to collect the data.
In discussing the concept of bandwidth, the operational consideration is the loss of energy that occurs when the signal is outside of the band, and as mentioned above, whatever losses occur can be made up for by increasing the data collection time. Thus, a practical definition of bandwidth is that all frequencies within the band can be sensed with the required sensitivity without increasing the data collection time by an appreciable fraction such as, for example, 10 percent. Thus, the bandwidth of a probe can be operationally defined as the frequency region over which its sensitivity is greater than 90 percent of the ideal sensitivity.
The present invention provides the desired broad-band sensitivity without requiring additional dampening resistances. The invention also provides the added benefit of matching the effective pick-up coil impedance to the receiver preamplifier input impedance in an optimum manner. In this fashion, the maximum possible amount of signal energy is transferred to the preamplifier input impedance while providing uniform sensitivity over a larger bandwidth. Briefly, the approach is to consider the pick-up coil as one of the inductances in a multipole impedance transforming bandpass network of one of the classical filter designs such as Butterworth, Chebychev, or Elliptic, for example.